Method of manufacturing Fe nanopowders by chemical vapor condensation

ABSTRACT

Disclosed is a method of synthesizing Fe powders having sizes of tens of nm by vaporizing an iron atom-containing liquid material with a low melting point at high temperatures, and then condensing iron atoms in decomposed Fe and CO gas by chemical vapor condensation. According to the current invention, the synthesizing method includes vaporizing iron pentacarbonyl (Fe(CO) 5 ) or iron acetate ((CH 3 CO 2 ) 2 Fe) precursor to gas by use of a ceramic bubbler of a chemical vapor condensation device, decomposing the vaporized gas to Fe in a reactor of the device while being introduced with Ar gas, and condensing the decomposed Fe in a chamber of the device, thereby obtaining the Fe powders having sizes of tens of nm.

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(a) to KoreanPatent Application No. 10-2003-0077589, filed on Nov. 4, 2003 by ByungKee Kim et al., entitled “METHOD OF MANUFACTURING Fe NANOPOWDERS BYCHEMICAL VAPOR CONDENSATION” the contents of which are herebyincorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to a manufacturing method of ironnanopowders, and more specifically, to a method of manufacturingnano-sized iron powders by means of chemical vapor condensation.

DESCRIPTION OF THE RELATED ART

Fine powders having magnetic properties, that is, fine magnetic powders,have various applications, for example, contrast media for magneticresonators, recording media for magnetic tapes, magnetic fluidmaterials, etc. The magnetic powders, presently commercially available,are exemplified by oxide-based powders, such as Fe₂O₃, Fe₃O₄, Fe-ferriteand Co-ferrite.

Conventionally, the magnetic powders have been mainly manufactured by aliquid reaction process, such as metal hydroxide reduction or metal saltreduction. In particular, the contrast medium having high quality andthe magnetic fluid for sealing materials are possible to be manufacturedby use of only material powders exhibiting superparamagnetic properties,which are fined to have a particle size not larger than a singlemagnetic domain. Accordingly, there required methods of synthesizingfiner magnetic metal powders having further decreased particle sizeswhile improving magnetic properties thereof.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to alleviate theproblems encountered in the related art and to provide a method ofsynthesizing metal iron powders having sizes of tens of nm, byvaporizing an iron atom-containing liquid material having a low meltingpoint at high temperatures and then condensing iron atoms in decomposedFe and CO gas by chemical vapor condensation.

To achieve the above object, the prevent invention provides a method ofmanufacturing Fe nanopowders by chemical vapor condensation, including:vaporizing a Fe-containing liquid precursor to gas, to obtain avaporized gas; decomposing the vaporized gas to Fe while beingintroduced with an inert gas, to obtain decomposed Fe; and condensingthe decomposed Fe, to obtain Fe nanopowders.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a schematic view of a chemical vapor condensation device usedin the present invention;

FIG. 2 is electron micrographs of Fe nanopowders manufactured atdifferent reaction temperatures;

FIG. 3 is an X-ray diffraction spectrum for the Fe nanopowders of FIG.2; and

FIG. 4 is enlarged electron micrographs of parts of the Fe nanopowders,in the Fe nanopowders of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, a detailed description will be given of the presentinvention, with reference to the appended drawings.

Useful in the present invention, a pure iron material has amagnetization value two or three times higher than that of oxidematerials, and also, has low anisotropic properties, thus having a lowercoercive force. Further, as a particle size of iron decreases, themagnetization value is constantly reduced while the coercive force isincreased, whereby iron is possible to be used as a magnetic recordingmedium. Moreover, iron having a very small particle size comes to be asuperparamagnetic material, and hence, is usable as a magnetic fluid.

To manufacture iron nanopowders of the present invention, there isrequired a chemical vapor condensation device, which is schematicallyshown in FIG. 1. As apparent from FIG. 1, the device (1) includes aceramic bubbler (3), a reactor (6), and a chamber (7).

A liquid precursor containing Fe is vaporized to gas by means of theceramic bubbler (3). That is, the liquid precursor in a storage bath (2)is fed through a feeding pipe (5) and a feeder (4), and then isvaporized while passing through the ceramic bubbler (3) that ismaintained at predetermined temperatures.

Suitable for use in the present invention, the Fe-containing liquidprecursor is exemplified by iron pentacarbonyl (Fe(CO)₅), or ironacetate ((CH₃CHO₂)₂Fe), in which iron pentacarbonyl having avaporization point of about 103° C. is easily vaporized at 150-220° C.

However, the gas vaporized by use of the ceramic bubbler (3) is notdecomposed to Fe and CO gas in the above temperature ranges. Therefore,while the vaporized gas is introduced with an inert gas, it passesthrough the reactor (6) which is maintained at high temperatures,whereby Fe is decomposed from the vaporized gas. In the presentinvention, the reactor (6) is in the temperature range of 400 to 1000°C., and preferably, 400 to 800° C. If the temperature of the reactor (6)is higher than 1000° C., large quantities of γ-Fe phase are obtained,together with α-Fe. In such cases, the γ-Fe phase as a non-magneticmaterial negatively affects requirement properties of synthetic powders,which is unfavorable.

The decomposed Fe gas transferred to the reactor (6) along with theinert gas is condensed to the size of tens of nm therein, and is formedto be crystalline Fe powders, which are then sprayed into the chamber(7). In the chamber (7), the crystalline Fe gas is floated for severalhours and then is attached to an inner wall or a bottom surface of thechamber (7). Even after the precursor solution is completely fed, thefloating of the Fe powders is continued for several hours in the chamber(7). The inert gas is continuously introduced into the chamber (7) untilall the synthesized Fe powders are stably settled down, whereby theinside of the chamber (7) is maintained in a non-oxidative protectionatmosphere and the CO gas remaining in a very small amount in thechamber (7) is discharged out of the chamber (7).

On the other hand, when the chamber (7) is opened as soon as thesynthesized Fe powders are collected therein, it may be exploded. Hence,before the chamber (7) is opened, a small amount of oxygen is fed intothe chamber (7) through an inlet (8) thereof, and thus, the ironnanopowders are coated with an oxide layer. Like this, it is preferredthat the iron nanopowders are subjected to passivation treatment to bestably handled under atmosphere.

A better understanding of the present invention may be obtained throughthe following examples which are set forth to illustrate, but are not tobe construed as the limit of the present invention.

EXAMPLE 1

Iron pentacarbonyl as a liquid precursor was fed into a chemical vaporcondensation device of FIG. 1, thus manufacturing Fe powders. As such, aceramic bubbler (3) of the above device was maintained in a range of150-200° C., and the liquid precursor was fed at 0.30 g/min. Then, whileAr gas was introduced at 2000 cc/min into the ceramic bubbler (3), thevaporized gas passed through a reactor (6) and then was sprayed into achamber (7). The reactor was formed with a virtually pure alumina tubehaving an inner diameter of 5 mm and a length of 300 mm, and was in thetemperature range of 400 to 1000° C.

In addition, before the chamber (7) was opened, air was introduced at2000 cc/min into the chamber (7), whereby an oxide coating layer wasformed on the respective Fe powders.

The manufactured Fe powders were observed by means of an electronmicroscope. The results are depicted in FIG. 2. As apparent fromelectron micrographs of FIG. 2, the Fe powders have an average particlesize of 8, 17 and 68 nm when the synthesizing temperatures are 400° C.(inventive example 1), 600° C. (inventive example 2) and 800° C.(inventive example 4). In addition, when the temperature of the reactorreaches 1000° C. (comparative example), the average particle size of theFe powders increases to 96 nm. Particularly, since the particle sizes ofthe powders synthesized at 400 and 600° C. are very fine to the extentof 20 nm or less, such powders may be agglomerated together. In thiscase, the agglomerated powders may be separated by use of energysources, such as ultrasonic waves and microwaves.

FIG. 3 shows analytic results of X-ray diffraction patterns of Fepowders according to reaction temperatures. As shown in FIG. 3, the Fepowders synthesized at 400° C. and 600° C., respectively, have anamorphous type peak of Fe₃O₄. Although the Fe powders synthesized at800° C. and 1000° C., respectively, have such an oxide, they have noX-ray diffraction peak. This is because the above oxide has a very smallvolume compared to the increased particle size of iron. In cases of theFe powders synthesized at 1000° C., large quantities of γ-Fe, which havea bad effect on the properties of the synthesized powders, are present,together with α-Fe.

FIG. 4 shows enlarged micrographs of the Fe powders synthesized at 600°C. and 800° C. among the Fe powders of FIG. 2, to observe the oxidecoating layer on the respective Fe powders. As seen in FIG. 4, the Fepowders synthesized at 600° C. and 800° C., respectively, have the oxidelayer which is 3-4 nm thick. That is, even though the surface oxidelayer is slightly thicker according to the increase of the reactiontemperature, it is hardly affected by the reaction temperature.

EXAMPLE 2

Fe powders were synthesized in the same manner as in Example 1, with theexception that the liquid precursor was fed at 0.15 g/min and reacted at600° C. (inventive example 3). The Fe powders had an average particlesize of 16 nm. From this, it can be found that a slow feeding rate ofthe precursor solution does not greatly affect the fineness of Fepowders.

EXAMPLE 3

The Fe nanopowders manufactured in Examples 1 and 2 were measured formagnetic properties, such as coercive force and maximum magnetizationvalue. The results are summarized in Table 1, below, along with processconditions of Fe powders. TABLE 1 Feeding Reactor Fe- Oxygen Fe₃O₄Coercive Max. Magnet. Rate Temp. Core Conc. Shell Force Value (g/min) (°C.) (nm) (wt %) (nm) (Oe) (emu/g) Note 0.30 400 8 14.3 2.5 745 125 Inv.Ex. 1 0.30 600 17 13.6 3.4 998 147 Inv. Ex. 2 0.15 600 16 13.5 3.4 1021145 Inv. Ex. 3 0.30 800 68 3.8 3.6 103 205 Inv. Ex. 4 0.30 1000 96 2.03.8 95 76 C. Ex.

Since the Fe powders manufactured at 1000° C. have a considerable amountof a nonmagnetic material, γ-Fe, magnetic properties thereof becomesinsignificant. However, as for the Fe powders synthesized attemperatures not higher than 800° C., the coercive force increases up to1021 Oe until the average particle size decreases to 16 nm, and then, isreduced again at the particle size of 10 nm or less. It is believed thatsuperparamagnetic properties appear on much smaller lo particles amongthe Fe powders having an average size of 8 nm. In common, the finer theparticles, the higher the spin non-arrangement effects based on increaseof surface areas. Hence, the maximum magnetization value may decreaseeven to about 50% or less of a maximum value of a bulk material. Forexample, a theoretical maximum magnetization value of bulk pure ironamounts to 225 emu/g, while the maximum magnetization value of iron ofthe present invention decreases from about 200 emu/g to about 120 emu/gby particle fineness.

As described hereinbefore, the present invention provides a method ofmanufacturing Fe nanopowders by chemical vapor condensation,characterized in that sizes, phases and magnetic properties of thesynthesized Fe nanopowders can be controlled according to reactiontemperatures. The Fe nanopowders of the present invention is applicableas a magnetic recording medium. Further, through process improvement,such as decrease of the reaction temperatures, the Fe powders can befurther fined, whereby they can be used as a magnetic fluid.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A method of manufacturing Fe nanopowders by chemical vaporcondensation, comprising: vaporizing a Fe-containing liquid precursor togas, to obtain a vaporized gas; decomposing the vaporized gas to Fewhile being introduced with an inert gas, to obtain decomposed Fe; andcondensing the decomposed Fe, to obtain Fe nanopowders.
 2. The method asdefined in claim 1, wherein the liquid precursor comprises ironpentacarbonyl (Fe(CO)₅), or iron acetate ((CH₃CO₂)₂Fe).
 3. The method asdefined in claim 1, wherein the decomposing of the vaporized gas isperformed at 400 to 800° C.
 4. The method as defined in claim 1, whereinthe condensing of the decomposed Fe further comprises feeding oxygen tocoat each surface of the Fe nanopowders with an oxide layer.